1. Field of the Invention
The present invention relates to a power source apparatus having a shunt resistor connected in series with the batteries to determine battery current by detecting the voltage induced across the resistor, and in particular to a power source apparatus optimally suited as a power source that is charged and discharged with high current such as a power source that supplies power to a motor to drive a vehicle.
2. Description of the Related Art
A power source apparatus is equipped with current sensing to detect battery charging and discharging current. A power source apparatus with a current sensor can integrate current to compute remaining capacity, can control battery charging and discharging using the computed remaining capacity, and can prevent battery over-charging and over-discharging. Batteries have the characteristic that they degrade significantly due to over-charging and over-discharging. Accordingly, accurate detection of the remaining capacity to prevent over-charging and over-discharging can effectively prevent battery degradation and lengthen battery lifetime. However, accurate detection of the battery current is critical for actual implementation. This is because battery current detection error results in battery remaining capacity error. In particular, since remaining capacity is computed from the integral of the current, current detection error can accumulate over time and remaining capacity error can gradually increases over time.
Since current is computed by the current sensor from an output voltage proportional to the electrical resistance of the shunt resistor, it is important to establish an accurate value for the electrical resistance of the shunt resistor to accurately detect current. In addition, wasted power proportional to the square of the current is consumed by the shunt resistor, and Joule heating occurs as a result of that power consumption. To limit wasted power consumption, the electrical resistance of the shunt resistor, which detects high currents, must be made small. Further, to efficiently dissipate heat, it is also important to make the resistor surface area large.
A shunt resistor made of metal plate has low electrical resistance and superior heat dissipating characteristics, and is ideally suited for high current detection. Therefore, the metal plate shunt resistor has been adopted for current sensors in high current power source apparatus such as vehicle power source apparatus.
A shunt resistor formed from metal plate and used in a vehicle power source apparatus is cited in Japanese Laid-Open Patent Publication 2004-117,045 and Japanese Laid-Open Patent Publication 2008-48,506.
The electrical resistance of a metal plate shunt resistor as cited in the references is determined by metal plate material, thickness, width, and length. The shunt resistor has current flow (forcing) terminals established at both ends of a metal plate of given length and width, and connecting leads are attached to those terminals for series connection with the batteries. Voltage detection (sensing) terminals are established at two points between the current flow terminals to detect induced voltage proportional to the current flow. The voltage detection terminals are connected to a current sensor difference amplifier, and the induced voltage is amplified by the difference amplifier to detect the battery current. The current sensor computes current from the induced voltage, which is assumed to be the product of the current and the electrical resistance of the shunt resistor. The electrical resistance used for current detection is not the resistance of the entire shunt resistor. More specifically, the electrical resistance is not resistance between the current flow terminals. Rather, for a metal plate shunt resistor provided with voltage detection terminals positioned inside the current flow terminals, an induced voltage is output that is the product of the current and electrical resistance between the pair of voltage detection terminals. The current is computed from that induced voltage. Since current is determined from the voltage induced between the voltage detection terminals, the shunt resistor is required to have an accurate and constant electrical resistance value between the pair of voltage detection terminals. The electrical resistance between the voltage detection terminals can be set according to the thickness, width, and length of the metal plate material between the voltage detection terminals. However, even when the metal plate material is fabricated to have constant dimensions between the voltage detection terminals, processing variation results in electrical resistance error. Electrical resistance error due to processing variation causes error in the induced voltage, which in turn results in error in the detected current. The electrical resistance of a shunt resistor made from metal plate can be adjusted by techniques such as shaving down the thickness or width of the metal plate between the voltage detection terminals. However, these techniques post-process shunt resistors manufactured in quantity to adjust their shapes, and consequently have the drawback that electrical resistance cannot be adjusted in a simple manner.
The present invention was developed to resolve the drawbacks described above. Thus, it is a primary object of the present invention to provide a power source apparatus that can finely adjust the actual electrical resistance between voltage detection terminals in an extremely simple and accurate manner without changing metal plate dimensions to allow accurate current detection with a shunt resistor.